ANAEROBIC THERMOPHILIC DIGESTION OF MANURE AT DIFFERENT AMMONIA LOADS - EFFECT OF TEMPERATURE

The effect of temperature in the range of 40-64 degrees C, on thermophilic anaerobic digestion of cattle manure with two different ammonia concentrations (2.5 and 6.0 g-N/l) was investigated in continuously-fed lab-scale reactors: the higher ammonia concentration reduced the maximum tolerable temperature. Poor process performance was observed when the combination of temperature and ammonia loading resulted in a calculated concentration of unionized ammonia (NH3) exceeding approx. 0.7 g-N/l, at the applied retention time of 15 days. When the ammonia load was high, reduction of the temperature below 55 degrees C resulted in an increase of the biogas yield and better process stability, indicated by a lowering of the concentration of volatile fatty acids in the effluent.